Intra- and inter-specific competition for winter concealment habitat in juvenile salmonids

The availability of streambed refuges has previously been shown to be of critical importance to the survival of overwintering juvenile salmonids. We used semi-natural stream channels to quantify intra- and inter-specific competition for daytime refuges and the willingness of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) to share available shelter. Refuge use was frequently associated with aggressive behaviour when two fish were provided with only one shelter, with intra- and inter-specific competition being similar in intensity. Resident individuals were less likely to leave the refuge than were intruders, and sharing of refuges was uncommon, both when competing for one shelter (experiment I) and when these were provided in excess (experiment II). Fish showed greater preferences for foraging habitat during the night than for the location of daytime shelters and were therefore willing to shift habitats to find adequate shelter. Overall, these results suggest that winter competition for refuges, both within and between species of salmonid, is likely to be intense if refuge availability is limited in the wild